Improper grounding of power distribution systems can have adverse effects upon the performance and maintenance of complex telecommunications systems, computer networking systems, and other electronic equipment. Such power distribution systems commonly consist of 120 volt, 60 hertz alternating current supplies in the U.S. and other western hemisphere countries, and 220 volt, 50 and 60 hertz supplies in Europe.
The power distribution systems which provide power to telephone switchboards, personal computers, facsimile machines, copy machines and other equipment are typically single phase, three conductor systems. Two of the conductors, a "high potential" or "hot" conductor and a "neutral" conductor, are current carrying. The third conductor, a "grounding" conductor, is typically bonded to the neutral conductor at the main service entrance of a building or at a major power distribution panel. At this point, the neutral and grounding conductors are also bonded to an "earth ground", which is typically a copper rod driven into the earth. Therefore, substantially no voltage should exist between the neutral conductor and any other conductor which is connected to earth, such as water pipes, metallic building structures, ventilating ducts, etc. The hot conductor will have a voltage potential with respect to such objects equal to the rated supply voltage.
In the United States, the National Electrical Code and the standards of Underwriters Laboratories (U.L.), and their counterparts in virtually all other countries, require that the metallic enclosures of any electrical utilization equipment be firmly bonded to the grounding conductor of a power distribution system.
A variety of "power protection" devices have been developed to safeguard electronic equipment from damage or system malfunction resulting from problems with the power distribution system. Among these are standby power supplies to supply power in the event of a power outage or malfunction. Other devices protect against transient voltage surges caused by lightning and power switching phenomena, or filter out high frequency "noise". Voltage regulators keep supply voltage within safe operating range for the equipment in the event of voltage sags or swells (i.e. surges) in the power distribution system.
Another problem associated with the power distribution system occurs when the resistance of the grounding conductor becomes unreasonably high. This may result from deterioration of the grounding conductor. Such deterioration can result from a number of causes including physical damage to the electrical conduit from construction work or impact of material handling equipment, slow deterioration from rust or exposure to corrosive chemicals, or loose connections caused by vibration or constant bumping of floor outlets by cleaning equipment. These problem do not usually occur with the neutral conductor which is protected by an outer casing. Therefore, the monitoring of the resistance between the neutral and grounding conductor is usually a good indication of the condition of the grounding conductor.
It is well accepted in the telecommunications and computer networking environment that a low resistance grounding system is a practical necessity. The connection of data lines between computers or other equipment which are separated by a considerable distance requires that the separated units be powered from different branch circuits in the power distribution system. If the "local" grounding conductors are at different potentials with respect to earth, the potential difference can be combined with data signals between the equipment to cause data errors or even damage to sensitive electronic devices driving and receiving the data signals Therefore, good installation practice stresses the importance of having lo resistance grounding conductors from each equipment site to a common earth point in the power distribution system.
The resistance of the ground path for a particular power distribution system branch circuit can be verified by measuring the resistance between the neutral and grounding conductors at an outlet of the branch circuit. Typically, this measurement may require that the power be shut-off to that branch circuit because a low resistance ohmmeter can be disturbed by line noise or voltage between the neutral and grounding conductors.
Other prior art methods of testing the condition of the grounding conductor have employed an indicator circuit connected between the hot conductor and the grounding conductor. To comply with U.L. shock hazard leakage requirements, the impedance of such circuits is necessarily high. Therefore, a grounding conductor path that is also a fairly high resistance will still indicate a satisfactory grounding conductor. Most such indicator circuits will show a satisfactory ground with a grounding conductor's path resistance of tens of thousands of ohms. All such circuits will indicate a satisfactory ground with resistance as high as 1000 ohms.
U.S. Pat. No. 3,810,003 discloses a technique for determining resistance of the grounding conductor path by measurement made at a power distribution system branch circuit outlet. The measurement is accomplished by intermittently pulsing a fairly high current (about 10 amperes) through the grounding conductor (i.e. a deliberate ground-fault current). The current flows through the hot conductor and the grounding conductor. The voltage drop through the grounding conductor can then be measured at the outlet between the neutral and grounding conductors. Since no pulse current is flowing in the neutral conductor, it will be at the potential of the junction of the neutral and grounding conductor at a power distribution panel. At the outlet, the potential of the grounding conductor will be raised by the voltage drop through the grounding conductor caused by the current pulse. Thus, the resistance of the grounding conductor can be measured utilizing the relationship R=V/I. This technique can be used to test the grounding conductor resistance. However, while proposals have been made of utilizing this device to continuously monitor the resistance of a grounding conductor, neither it nor an ohmmeter can be used as a continuous monitoring device since neither can meet the U.L. leakage current safety requirement of 3.5 milliamperes. This leakage requirement is the maximum current which may flow between the grounding and neutral conductors in the event such conductors are accidentally reversed at an outlet.
Moreover, the 3.5 milliampere leakage current limit is too low to make that current useful for detecting a variation of several ohms in the current path because it would require resolving a voltage change of the order of 10 millivolts. This is impractical because there is always some voltage appearing between the neutral and grounding conductors from noise, transient surges or load-current drop in the resistance of the neutral conductor. For instance, a 1000 watt, 70% power factor typical computer load can result in peak current through the neutral conductor of over 30 amperes. This would cause a 3 volt drop in the neutral conductor with a resistance of only 0.1 ohm. Since this voltage is added to the small measuring current voltage drop in the grounding conductor, it would obscure the 10 millivolt or less change that must be detected. Thus, a monitoring device which measures the voltage drop through the grounding conductor and is confined to the branch circuit outlet to be monitored, cannot operate within the applicable safety standards.
U.S. Pat. No. 4,973,912 discloses a method and device for contactless measurement of resistance arranged in the secondary circuit of a transformer. The time constant for a decaying sinusoidal response is employed to determine a value of an unknown resistance. The device comprises an excitation circuit for exciting a tuned circuit containing an unknown resistance to be measured and a time constant waveform analyzer to determine the time constant of the resulting response to the excitation signal. The patent suggests that various methods may be employed to measure the time constant. However, a method of counting the peaks of the decaying sinusoidal response until the voltage reaches a certain value is specifically taught. The device is utilized to monitor the resistance of the firing cap of the automotive airbag system located in the steering wheel of the vehicle. There is no teaching or suggestion that this device is useful as a monitoring apparatus for the grounding to neutral circuit resistance of a power distribution system or how it could be associated with the hot, grounding and neutral conductors of such a system.
The unique method of the present invention can continuously and reliably monitor the resistance of a grounding conductor while coupled to a power distribution system circuit. Furthermore, this method may be embodied in an apparatus which complies with the applicable safety U.L. and other standards.